The information superhighway will primarily comprise optical fibers for the foreseeable future because of the enormous bandwidth that each optical fiber provides. For example, a typical optical fiber exhibits relatively low loss over the wavelength region 820–1600 nm. This particular region provides a bandwidth of about 180,000 GHz which means that a single optical fiber can potentially carry 45 billion voice channels (4 kHz each) or 30 million television channels (6 MHz each). And while these numbers represent upper limits that are not practical to attain, they provide a compelling reason for communication carriers to use optical transmission.
In order to fully utilize this information superhighway, there is a need to filter a group of signal channels such that these channels can be further separated, redirected to a different direction, or a fraction of these channels be dropped and added. In certain applications, interleavers and de-interleavers are used to bridge technologies with different channel spacing, enabling the usage of more economical solutions associated with larger channel spacing. For example, various prior art interleavers based on a Gires-Toumois (GT) mirror and a Michelson interferometer separate a composite input optical signal into two complementary signals in which the odd data channels are branched into one output and the even channels are directed back into the input.
In other proposed prior art interleaver designs an input signal is coupled to a 50% non-polarizing cubic beam splitter through a collimating lens such as a graded index lens (GRIN) lens. A GT mirror and a regular mirror are used to form an interferometer. The odd channels return to one output fiber through another lens whereas the even channels return to the input fiber through a lens.
In yet other prior art interleaver designs based on a polarization beam splitter (PBS) and two GT mirrors an input signal is coupled to a PBS through a collimating lens. The two arms of the device are two interferometers, one for each of the polarization components. For each interferometer, a polarization and phase-modified GT mirror is used as two mirrors of the interferometer. The phases and Free Spectra Ranges (FSR) of the GT mirrors are modified/adjusted using waveplates. The relative phases of the two paths of each of the interferometers are adjusted by changing the orientations and thickness of the waveplates. Both interferometers are adjusted such that the odd channels return to one output fiber through the first lens whereas the even channels return to the other fiber through another lens.
For all of the advantages of the prior art interleaver designs, there are several areas of improvements needed. For instance, the use of a Michelson interferometer with one output returning to the same direction in requires the use of an optical circulator in the optical “circuit” in order to physically separate the output from the input. This increases the cost and form factor of such a design. Another area of improvement is in the temperature stability of the prior art devices. Device not based on a balanced design will require temperature stabilization whereas devices using thin glass plates/wave plates for fine adjustments of the interferometers introduce reliability issues such as the use of epoxy and certain temperature related drifts.